The present disclosure relates to semiconductor memory devices and, more particularly, to a fuse circuit using nonvolatile memory cells.
Semiconductor devices (or semiconductor chips) usually employ fuses for storing optional information for the purpose of altering design patterns or selecting specific options after the manufacturing of their chips. It is generally known that fuse circuits simply operate to connect or disconnect signals or power lines, relevant to functional circuits, to and from each other. Furthermore, it is possible to store required information or modify design configurations by combining connections and disconnections with the fuse circuits.
One of the typical methods for implementing fuse circuits is to utilize laser fuses. In this kind of fuse circuit, fuses are formed by a pattern of polysilicon bars and are melted away by irradiating a laser thereon according to the particular needs. More specifically, both ends of the polysilicon fuse, which is made of a conductive material, are initially electrically connected together and then disconnected after the laser cuts the connection therebetween, or are electrically isolated from each other. By way of such a procedure, the fuse circuit is controlled between the state of connection and disconnection. On the other hand, it is restrictive such that the cut-off operation should be carried out by the laser during a wafer-level test before packaging. Furthermore, the laser fuses are usually cut off after the chip moves to special laser equipment that is different from the equipment used for evaluating the electrical characteristics of the chip, which also incurs an increase of a test time. A further problem is that once the fuses are cut off, there is no way of recovering their original connections due to the physical property of a laser fuse. Moreover, the laser fuses occupy an area larger than a predetermined space relative to the actual circuit on the chip and need a pad option for cutting, so that they occupy a large layout area on the semiconductor chip.
In an approach to overcoming the aforementioned inconvenience involved in laser fuses, there are mostly used electric fuse circuits formed of nonvolatile memory cells. Generally, an electric fuse circuit includes a pair of nonvolatile memory cells that are maintained in a complementary condition with each other through an electrical erasing or programming operation. As the electric fuse circuit retains information about options in the complementary relation with threshold voltages of the two nonvolatile memory cells, it is highly important for the circuit to retain its original information, even for a relatively long time. More specifically, one of the nonvolatile memory cells should be conditioned to have its threshold voltage lower than a reference level, even after a time, while the other should be maintained with its threshold voltage higher than the reference level. Thus, it is properly essential to estimate preservability of information in the electric fuse circuit even after a relatively long time has elapsed.